For supplying a hydraulic fluid delivered from a hydraulic pump to a plurality of hydraulic actuators, it is conventional to provide a plurality of directional control valves in a delivery line of the hydraulic pump and shift the directional control valves selectively so that the hydraulic fluid is supplied to desired one of the hydraulic actuators. With this arrangement, however, when the hydraulic fluid is supplied to several hydraulic actuators at the same time, the hydraulic fluid is supplied to only the hydraulic actuator under a smaller load, and no hydraulic fluid is supplied to the hydraulic actuator under a larger load.
JP, B, 4-48967 and U.S. Pat. No. 5,305,789, for example, propose hydraulic circuits for overcoming such a problem.
In the hydraulic circuit proposed in JP, B, 4-48967, a plurality of directional control valves are provided in a delivery line of a hydraulic pump, and a pressure compensating valve for varying a setting differential pressure depending on a load sensing differential pressure (i.e., a differential pressure between a maximum load pressure among the plurality of directional control valves and a delivery pressure of the hydraulic pump) is provided in a circuit section between the hydraulic pump and a variable throttle of each of the directional control valves. The pressure compensating valve controls a differential pressure across the variable throttle.
In the hydraulic circuit proposed in U.S. Pat. No. 5,305,789, a plurality of directional control valves are provided in a delivery line of a hydraulic pump, and a pressure control valve responsive to a maximum load pressure is provided in a circuit section between a variable throttle of each of the directional control valves and each corresponding hydraulic actuator. The pressure control valve controls a pressure on the outlet side of the variable throttle to be kept substantially at the maximum load pressure.
Hereunder, the pressure compensating valve disclosed in JP, B, 4-48967 is referred to as the prepositional type and the pressure control valve disclosed in U.S. Pat. No. 5,305,789 is referred to as the postpositional type. Further, the prepositional type pressure compensating valve is referred to as a variable pressure compensating valve and the postpositional type pressure control valve is referred to as a flow distribution valve.
To effect the function of the variable pressure compensating valve or the flow distribution valve, the maximum load pressure is detected with a shuttle valve or the like and introduced to a signal line.
FIG. 12 shows the hydraulic circuit proposed in JP, B, 4-48967. A maximum load pressure detected by a shuttle valve 237 is output to a signal line 238. The maximum load pressure is transmitted through signal lines 239, 241 from the signal line 238 to one ends of variable pressure compensating valves 206, 216 provided respectively between a hydraulic pump 201 and directional control valves 208, 218.
When the maximum load pressure is transmitted in such a way, the variable pressure compensating valves 206, 216 operate so that the relationship of;
(pump pressure in line 240)--(maximum load pressure in line 239)=(pressure in line 225 upstream of variable throttle)--(pressure in line 224 downstream of variable throttle) PA1 (pump pressure in line 242)--(maximum load pressure in line 241)=(pressure in line 227 upstream of variable throttle)--(pressure in line 226 downstream of variable throttle) PA1 (pump pressure in line 240)=(pump pressure in line 242), and (maximum load pressure in line 239)=(maximum load pressure in line 241)
holds on the side of the directional control valve 208 and the relationship of;
holds on the side of the directional control valve 218.
Because of;
differential pressures across the variable throttles of the directional control valves 208 and 218 are equal to each other.
Accordingly, even when a difference in load pressure exists between hydraulic actuators 212 and 222, a delivery rate of the hydraulic pump 201 is distributed depending on an opening area ratio between the variable throttles, and hence a hydraulic fluid is avoided from being supplied to the hydraulic actuator under a smaller load pressure with priority.
FIG. 13 shows the hydraulic circuit proposed in U.S. Pat. No. 5,305,789, and FIG. 14 shows one example of valve structure. A modification of the valve structure is shown in FIG. 15.
In FIGS. 13 and 14, a flow distribution valve 314, which serves also as a shuttle valve for detecting a maximum load pressure, is disposed between a directional control valve spool 304 and ports A, B connected to each hydraulic actuator. The maximum load pressure detected by the flow distribution valve 314 is introduced to a signal line 308 and then to other flow distribution valves 314 associated with corresponding directional control valves. With this arrangement, the flow distribution valve 314 on the side of the actuator under a lower load pressure is not opened until the pressure in an inlet line 312 of the flow distribution valve 314 becomes equal to the detected maximum load pressure in the signal line 308.
When a plurality of directional control valves associated with respective hydraulic actuators under load pressures different from each other are operated at the same time, the pressures in the inlet lines 312 of the flow distribution valves 314 associated with the operated directional control valves are all equal to the maximum load pressure. As a result, differential pressures across variable throttles of the directional control valve spools 304 are equal to each other in all the directional control valves. In this prior art, therefore, a delivery rate of a hydraulic pump is distributed depending on an opening area ratio between metering notches (variable throttles) 320 regardless of the magnitudes of load pressures as with the above prior art.
In the postpositional type arrangement, it is general that the flow distribution valve 314 is provided one for each directional control valve, as shown in FIGS. 13 and 14. FIG. 15 shows an example wherein two flow distribution valves are provided for each directional control valve in the postpositional type arrangement. In FIG. 15, because metering notches 320 formed on a spool 304 have functions of both flow rate control and direction control, the hydraulic fluid having passed the flow distribution valve 314 is supplied to the port A or B without passing the spool section again.